It is known in the design of line echo cancellers which utilize adaptive filters to incorporate a non-linear processor (NLP) for removal of residual echo signals (e.g. due to non-linearity, distortion, or added signal noise). In order to avoid noise switching being heard on the far end side, it is important that the noise level of the signal applied to the NLP be calculated to distinguish between noise and residual echo.
Noise level calculation is also useful in determining if the reference signal applied to the adaptive filter is noise or a non-noise segment. If noise is detected, then updating of the filter coefficients may be suppressed.